1. Field of the Invention
The present invention relates to a contact charger for use in an electrophotographic image forming apparatus such as a copying machine or a printer, and also relates to an image forming apparatus provided with such a contact charger.
2. Description of Related Art
<Corona Charger>
Conventional image forming apparatuses such as an electrophotographic device have employed corona chargers, which utilize corona discharging for charging a charging target (i.e., an object to be charged) such as a photosensitive member for electrophotography.
The corona charger is arranged in a noncontact manner with respect to the charging target, and is configured such that a high voltage is applied, e.g., to a wire electrode or a needle electrode for causing corona discharging, and thereby a part of discharge current thus caused flows through the charging target to place a predetermined potential on the charging target.
However, the corona charger utilizing the corona discharging generates a large amount of ozone, which causes a problem due to ozone smell or the like. Also, a discharging product produced by the corona discharging adheres to a surface of the charging target. Thereby, quality of images is impaired, and/or the surface of the charging target has to be shaved for recovery from deterioration due to the adhesive discharging product.
This causes problems such as reduction of durability of the charging target. Further, a power source of a high voltage and therefore an expensive power source are required.
<Contact Charger (Charging by Discharging)>
In recent years, therefore, many contact chargers have been proposed for use instead of the corona chargers. For example, a roller charger, a fur-brush charger, a blade charger and others have been proposed. These charges are configured to charge the charging target by utilizing a discharging phenomenon, which occurs between the charging target and the charging member. The charging member is arranged in direct contact with the charging target, and a voltage is applied to the charging member to place a predetermined potential on the charging target.
The roller charger includes an elastic roller having, e.g., an electrically conductive elastic layer. The elastic roller is in contact with the charging target to form a nip, and a voltage is applied to the elastic roller to charge the charging target. In many structures, the elastic roller is driven to rotate by the charging target.
The fur-brush charger is formed of a fur-brush roller, e.g., having electrically conductive brush fibers. The fur-brush roller is in contact with the charging target to form a nip, and a voltage is applied to the fur brush to charge the charging target.
Since the fibers used therein are extremely thin, a strong electric field is locally produced between the fur brush and the charging target, and excessive discharging not following Paschen's law occurs in the strong electric field so that irregular charging occurs.
Since the contact between the charging target and the brush fibers consists of a gathering or combination of line-contacts and/or point-contacts, it is difficult to ensure a sufficiently large contact area between the charging target and the fur brush so that it is impossible to prevent insufficient charging due to insufficient contact.
These contact chargers can charge the target with power sources of lower voltages than those of the corona charger. In these contact chargers, however, a voltage prepared by adding a threshold voltage to an intended charging potential for following Paschen's law must be applied to the charging member. Further, the amount of produced ozone can be smaller than that of the corona charger, but disadvantages due to the discharging product are unavoidable because the charging operation utilizes the discharging phenomenon.
<Contact Charger (Injection Charging)>
For overcoming the above problems, such a contact charger has been proposed that injects electric charges directly into a charging target without utilizing the discharging phenomenon. For example, a magnetic brush charger, a roller charger, a fur-brush charger and others have been proposed as the contact chargers utilizing injection charging.
These chargers are configured to charge the charging target to bear a voltage substantially equal to the voltage applied to the charging member, and therefore can utilize a voltage lower than that of the foregoing contact charger utilizing the discharging phenomenon. Further, the discharging does not occur or is sufficiently suppressed so that the discharging product hardly occurs, and disadvantages due to the discharging product do not occur.
The magnetic brush charger is formed of, e.g., a nonmagnetic sleeve covering a magnetic roller and magnetic carriers retained on the sleeve, which hold electrically conductive particles. Spikes (magnetic brush) formed of the carriers holding the conductive particles are in contact with the charging target to form a nip, and a voltage is applied to the magnetic brush to charge the charging target by charge injection. This type of charger requires a complicated structure, and therefore is expensive. Further, it suffers from dropping of the magnetic carriers as well as image noises due to adhesion of the magnetic carriers onto the charging target such as a photosensitive member.
According to the roller charger, the conductive and elastic roller is brought into contact with the charging target to form a nip, and a voltage is applied to the elastic roller to effect injection charging on the charging target. For effecting the injection charging on the charging target, a sufficient contact area is required between the roller surface and the charging target.
However, such a sufficient contact area cannot be achieved if the elastic roller is merely driven to rotate by the charging target. For obtaining the sufficient contact area, a difference may be provided between peripheral speeds of the elastic roller and the charging target so that the elastic roller may slide on the charging target. However, this causes a large frictional force because the elastic roller is in face-contact with the charging target. Thereby, the surfaces of the charging member and the charging target may be unnecessarily shaved to generate image noises. Also, the durability thereof may be reduced.
For reducing the frictional force, Japanese Laid-Open Patent Publication No. H10-307458 has disclosed a roller charger, in which conductive particles are disposed in a contact nip between the roller charger and the charging target.
Even in this structure, a frictional force is larger than that in the chargers, which utilize line-contact and/or point-contact of a fur-brush or a magnetic brush, and therefore, the charging member and the charging target are shaved so that image noises occur, and low durability is unavoidable.
For example, Japanese Laid-Open Patent Publication No. H10-307457 has disclosed a fur-brush charger, in which a fur brush is in contact with the charging target to form a nip, conductive particles are present in this nip portion at a rate of 102 pcs/mm2 or more, and a voltage is applied to the fur brush to perform injection charging on the charging target.
Since the fur brush is in line-contact and/or a point-contact with the charging target, a frictional force between them is small, and wearing of the charging member and the charging target is considerably suppressed. Further, the discharging phenomenon is not utilized so that irregular charging due to excessive discharging can be prevented.
Since the conductive particles are present between the charging target and the fur brush, insufficient contact between the fur brush and the charging target can be suppressed, as compared with the fur brush charging utilizing the discharging phenomenon already described.
However, the fur brush injection charging device, in which the conductive particles are present in the contact nip portion between the fur brush and the charging target, suffers from a problem that stable charging cannot be sufficiently performed because the conductive particles drop from the fur brush.
In connection with the above problem, Japanese Laid-Open Patent Publication No. H11-190930 has disclosed a technique, in which conductive particles are mixed into developer to be supplied. Also, U.S. Pat. No. 6,233,419 has disclosed a technique, in which a conductive particle supply member such as an elastic foam roller or a fur brush is used for supplying conductive particles.
However, it is difficult to utilize sufficiently the technique, in which the conductive particles are mixed into the developer as disclosed in Japanese Laid-Open Patent Publication No. H11-190930, because the chargeability of the toner may lower depending on the mixing rate of the conductive particles into the developer and/or depending on the particle diameter of the conductive particles. If the supply member is used for supplying the conductive particles, as disclosed in U.S. Pat. No. 6,233,419, the supply member increases a cost.